Us vs universe: Unfuzzying the uncertainty principle

Video: Why uncertainty is useful in quantum physics

There is a loophole in Heisenberg’s quantum uncertainty principle – and we’re squeezing light through it to detect gravitational waves

IN FEBRUARY 1927, a young assistant to the quantum pioneer Niels Bohr had a brainwave about why measuring electrons provided consistently fuzzy answers. “The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa,” he wrote.

Werner Heisenberg had discovered his famous uncertainty principle – a cornerstone of quantum physics that limits how well we can know not just position and momentum, but a whole host of “complementary” pairs of quantities.

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Uncertainty has practical consequences. Take gravitational waves, which are a key unverified prediction of Einstein’s general theory of relativity. Experiments to detect these ripples in space-time rely on measuring tiny disturbances to laser light bouncing between distant mirrors. “People believed there was a fundamental limit for the sensitivity of such detectors, the standard quantum limit,” says Karsten Danzmann of the Max Planck Institute for Gravitational Physics in Hanover, Germany, who works on one such detector, GEO 600.

In fact there is a loophole. Uncertainty says you can learn more about one quantity by knowing less about its complementary quantity. To detect high-frequency gravitational waves, you need to know accurately the number of photons hitting the mirrors, but are less concerned about its complementary quantity, their arrival times. Squeeze all the uncertainty into the photons’ timing, and you can increase your detection-sensitivity dramatically.

“Squeezed light” was first proposed by physicist Carlton Caves in the 1980s, but it is only in the past decade that physicists mastered the necessary techniques, which involve splitting one normal ...

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